Pivotable mount for electric field electrodes of a cycloidal mass spectrometer



Aprll 15, 1969 R. A. 'ERICKSON 3,439,162

EIVQTABLE MOUNT FOR bLI:.CTRI.C FIELD ELECTRODES OF A CYCLOIDAL MASS SPECTROMETER Filed March 24, 1966 Sheet 1 of 2 PUMP GAS SOURCE AMPLIFIER RECORDER SCAN GENERATOR A ER VOLTAGE SUPPLY I I 33' *53 w Q FIGZ I lay/Fi "5 ll 1' INVENTOR.

RAYMOND A. ERICKSON ORNEY I BY 402 M 3 a a TT 3,439,162 LECTRODES Sheet Q or 2 R. A. ERICKSON PIVOTABLE MOUNT FOR-ELECTRIC FIELD E OF A CYCLOIDAL MASS SPECTROME INVENTOR 259'? A. ERICKSON 7144- RNEY United States Patent 3,439,162 PIVOTABLE MOUNT FOR ELECTRIC FIELD ELECTRODES OF A CYCLOIDAL MASS SPECTROMETER Raymond A. Erickson, San Jose, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Mar. 24, 1966, Ser. No. 537,188 Int. Cl. B01d 59/44 US. Cl. 250-413 8 Claims ABSTRACT OF THE DISCLOSURE A cycloidal mass spectrometer is disclosed. The spectrometer includes an evacuated envelope disposed between the pole pieces of a powerful electromagnet and containing an array of axially aligned parallel elongated ringshaped electrodes for producing a region of electric field at right angles to the DC. magnetic field produced by the magnet. An ion source is contained within the envelope for forming and projecting a stream of ions to be analyzed into the region of crossed electric and magnetic fields for mass analysis of the ions. The envelope includes a demountable flange having an electrode support structure for the rings projecting therefrom. The ring electrodes are pivotably mounted from the support structure such that separate ones of the ring electrodes are pivotable out of the array of electrodes for maintenance and pivotable back into an operating position in the array without loss of alignment.

Heretofore cycloidal mass spectrometers, of the type described in US. Patent 2,221,467, issued Nov. 12, 1940, have been built using an array of rectangular ring shaped electric field producing electrodes in the ion analyzer assembly. Typically, the rings have been spaced apart by dielectric washer type spacers ground to the desired thickness. The ring electrodes and spacers were mounted on support rods threading through the successive spacer and ring electrode members with nuts threaded over the ends of the support rods to clamp the analyzer assembly together. The problem with this type of a mounting assembly is that it must be essentially completely disassembled for cleaning or for gaining access, as for cleaning or ad justment, to the ion source and ion detector assemblies buried inside the array. It turns out that the inside surfaces of the array of electric field electrodes require frequent cleaning by abrasive methods such as sand blasting or rubbing with a crocus cloth. Un-ionized gaseous substances emerging from the ion source condense upon the field electrodes in the vicinity of the ion source. These substances build up an electrical insulating layer on the inside surfaces of the electrodes. The insulative layer allows build up of spurious surface charges that perturb the desired degree of uniformity of the applied electric field of the analyzer. The output mass number resolution of the spectrometer is directly proportional to the uniformity of the electric field in the analyzer and thus a high resolution output, i.e., greater than 1000 requires that the electrodes be maintained free of such contamination. As a consequence the electrodes require frequent cleaning. When the analyzer electrodes must be disassembled for cleaning, replacing the electrodes, with the required degree of precision, is a time consuming and troublesome effort.

In the present invention the analyzer electric field producing electrodes are pivotably mounted in a kinematic mount whereby the electrodes may be swung out of the array to permit cleaning and to gain access to the ion source and detector assemblies for adjustment or cleaning. After cleaning, the electrodes are merely swung back into proper position for operation.

3,439,162 Patented Apr. 15, 1969 The principal object of the present invention is the provision of an improved cycloidal mass spectrometer characterized by ease of cleaning and adjustment.

One feature of the present invention is the provision of a pivotable kinematic mount for the array of ion analyzer electrodes to facilitate their cleaning and to facilitate cleaning and adjustment of the ion source and detector assemblies.

Another feature of the present invention is the same as the preceding feature wherein the electrodes are pivotably mounted on a precision ground and aligned dielectric rod extending the length of the electrode array.

Another feature of the present invention is the same as any one or more of the preceding features including an indexing block disposed along one side of the electrode array for indexing the ends of the separate electrodes of the array to maintain their precise parallelism when they are pivoted back into their operating positions.

Another feature of the present invention is the same as any one or more of the preceding features including the provision of electrode support rods disposed along the opposite ends of the electrodes, supporting same, and extending from a support flange structure in the direction of the line of development of the electrode array to provide rigid support for the array.

Another feature of the present invention is the same as any one or more of the preceding including the provision of an array of cantilever spring electrodes for applying operating potentials to the electrode array.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a fragmentary side elevational view, partly in schematic form, of a cycloidal mass spectrometer employing features of the present invention.

FIGS. 2 and 3 are enlarged fragmentary views of portions of the structure of FIG. 1 delineated by lines 22 and 3-3, and

FIG. 4 is a sectional view of the structure of FIG. 1 taken along line 44 in the direction of the arrows and showing a certain electrode swung open.

Referring now to FIG. 1 there is shown a cycloidal mass spectrometer system. More particularly, an array of generally rectangular shaped ring electrodes: 1 are insulatively supported within a thin rectangular vacuum envelope 2, only partially shown, from a heavy demountable rectangular flange portion of the envelope 2 which closes off one end of the vacuum envelope 2.

The separate rings 1 of the electrode array are operated at slightly different electric potentials derived from 21 voltage source 3 via cable leads 4 connected at nodes of a voltage divider network inside the voltage supply 3. The different potentials applied to the different rings 1 establishes a region of uniform electric field E in the hollow interior of the ring electrode array. The electric field E is directed parallel to the line of development of the ring electrode array.

The electrode array is immersed in a uniform region of magnetic field H directed at right angles to the direction of the electric field E. The field H is conveniently produced by an electromagnet 7 with the vacuum envelope 2 being disposed in the narrow gap defined between a pair of pole piece 88 of the magnet 7.

The envelope 2 is evacuated in use via pump 10 to a suitably low pressure as of 10- torr. Gas to be analyzed by the analyzer section, including the array of electrodes 1, is introduced from a source 9 into the analyzer section through the vacuum envelope 2 via an inlet tubing 11 as of stainless steel. The inlet tubing 11 feeds gas at a desired rate into an ion source 12. The ion source ionizes the gas 3 and projects it through a slot into the crossed magnetic field H and electric field E of the analyzer.

Under the influence of the crossed electric and magnetic fields the ions are caused to execute cycloidal trajectories. However, only ions of a certain mass number, for a given intensity of E and H, will be focused at a detector slot 13 a certain focal distance from the source and at the same electric potential. An ion detector 14 is positioned behind the slot 13 to produce an output corresponding to number of ions under analysis having the certain predetermined focused mass number, if any.

The output is fed to an amplifier 15 which amplifies the detected signal and feeds it to the Y axis of an X-Y recorder 16 wherein it is recorded as a function of a scan of the magnetic field intensity H produced by a scan generator 17. The output of the recorder 16 is a mass spectrum of the sample under analysis.

The rectangular rings 1 are elongated to define a major axis 21 in the direction of elongation, see FIG. 4, and a minor axis 22 parallel to the magnetic field H. The rings 1 are made of stainless steel and have a marginal thickness 1 and width w of 0.250"i0.001, a length l of 7" and an inside width W along the minor axis of 1". The rings 1 include marginal mounting tab portions 23 and 23' at both ends for mounting to a support structure.

The electrode support structure includes three parallel directed heavy rods 24, 25 and 26 as of stainless steel welded at their ends to the heavy rectangular flange 2' as of 0.750" thick stainless steel. The lower support rod 24 as of 0.750" diameter stainless steel serves to support a precision ground dielectric hinge pin or rod 27 as of alumina ceramic via the intermediary of a pair of axially spaced V-blocks 28 and 29 pinned to the support rod via stainless steel pins 31 passing transversely through the support rod 24.

The dielectric hinge pin 27 as of 0.375" diameter is clamped via clamps into the V portion of the V-block and passes through aligned precision bores 32 in the mounting tab portions 23 of the ring electrodes 1. Dielectric washers 33 as of alumina ceramic (see FIG. 2) are precision ground to a predetermined thickness as a 0.050 +-0.001 and positioned on the rod 27 inbetween adjacent mounting tab portions 23 of the ring electrodes 1.

A clamping nut assembly 34, comprising a traveling nut 35 threadably mating with a threaded ring 36, is positioned on the hinge pin 27 between one of the V-blocks 29 and the last washer 33 of the electrode array. Backing off the nut 35, puts a compressive force F on the array of washers and ring electrodes 1 forcing them into a tight interference fit against the other V-block 28 to assure proper spacing between adjacent ring electrodes 1.

Electric operating potentials are applied to a number of the outside ring members 1 via an array of cantilever spring wires 37 (see FIG. 4) as of mil Elgiloy stainless steel spring wire, as manufactured by Elgin Watch Company, having contact balls 38 crimped on the free ends thereof. The contact balls 38 ride in sliding engagement with the mounting tab 23. The spring wires 37 are held to the support rod 24 by being clamped between two dielectric slabs 39 carried from the support rod 24.

In opening the electrode array for maintenance, the ring electrodes are separately free to pivot on the hinge pin 27, i.e., about an axis defined by the hinge pin and parallel to the line of development 41 of the ring electrode array. As the ring electrodes 1 swing open the electrical spring contacts 37 disengage the ring 1, as shown in phantom lines in FIG. 4. In the open position, the inside surfaces of the rings 1 may be easily cleaned as by rubbing with crocus cloth. Also when the rings 1 are in the open position they readily permit access to the ion source 12 and detector assemblies 14 mounted within the electrode array.

The upper ends of the outer ring electrodes 1 are supported from the upper support rod 26, as of 0.750" by 0.500" cross section stainless steel, via the intermediary of a dielectric indexing block 42 held to the rod via mounting screws 43. The indexing block 42 as of alumina or boron nitride is provided with an array of precision formed recesses 44 (see FIG. 3) which receive the upper mounting tabs 23' of the rings 1. The lands 45 between adjacent recesses 44 have a predetermined precision thickness as of 0.050i0.001 corresponding to the spacing between adjacent rings 1. Mounting screws 46 pass through holes in the upper mounting tabs 23 and thread into tapped holes in the indexing block 42 for holding the rings 1 to the upper rod 26 of the support structure.

The shorter rings 1, at the inside end of the ring array, are similarly mounted via another dielectric indexing block 47 to the center support rod 25 as of 0.750" by 0.500" cross section stainless steel. Electric operating potentials for the shorter rings 1 are brought to the upper ends of the rings via an array of leads 48 interconnecting the cable 4 and the upper mounting screws 46.

The pivotable kinematic mount for the ring electrodes 1 permits them to be swung open for maintenance and then swung back and fastened in place within their respective recesses 44, in the indexing blocks 42 and 47, while maintaining the predetermined precise alignment and parallelism to $0.001". To effect maintenance of the analyzer, the flange 2' with its dependent electrode array is removed from thevacuum envelope 2 by unfastening its demountable flange joint. Maintenance is performed and the flange 2 is remounted and the vacuum system processed by baking and pumping to the operating pressure range of 10- torr.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter conained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a cycloidal mass spectrometer, means forming an array of axially aligned parallel elongated ring-shaped electrodes for producing a region of electric field at right angles to a DC. magnetic field, means for projecting a stream of ions to be mass analyzed into the region of crossed electric and magnetic fields for mass analysis thereof, means forming a demountable flange, means forming a support structure projecting from said flange, the improvement comprising, means for separately pivotably mounting said ring-shaped electrodes of said array from said support structure such that separate ones of said electrodes are pivotable out of said array for maintenance and pivotable back into an operating position in said array without loss of alignment.

2. The apparatus according to claim 1, wherein said elongated ring electrodes each have an opening with a major and a minor axis, and means for supporting said rings of said array at both ends of their major axes.

3. The apparatus according to claim 3, wherein said support structure include a rod passing through aligned apertures in the end marginal portions of said rings, said rings being pivotable about the axis of said rod, and means forming washer-like-spacers on said rod inbetween adjacent rings to determine the spacing between adjacent rings.

4. The apparatus according to claim 3, wherein said rod and said spacers are made of an electrically insulative material.

5. The apparatus according to claim 4, wherein said support structure includes a metallic support rod parallel directed of said insulative rod, a pair of axially spaced V-blocks carried from said metallic rod, and said insulative rod being supported within the V-portions of said V-blocks.

6. The apparatus according to claim 3 including, means for axially compressing said rings and said spacers on said rod.

7. The apparatus according to claim 3 including, means forming an array of cantilever spring wires disposed at one end of said ring electrodes for applying the operating electrical potential to said ring electrodes by engaging said rings when said rings are in the operating position and disengaging said rings when said rings are pivoted out of the array.

8. The apparatus according to claim 3, wherein said support structure includes a second support rod disposed at the opposite end of said rings from said first mentioned support rod, means forming an indexing block made of an insulative material and mounted on said second support rod, said indexing block having an array of separate recessed portions for receiving and indexing separate ones of said rings and for providing alignment for said array of rings.

References Cited UNITED STATES PATENTS 2,221,467 11/1940 Bleakney 25041.9 2,619,609 11/1952 Reid 250-41.9 2,871,361 11/1959 Andrews 250-41.9

RALPH G. NILSON, Primary Examiner. S. C. SHEAR, Assistant Examiner. 

